Detrital Dynamics in a Mature Oak Forest: Hutcheson Memorial Forest, New Jersey

The William L. Hutcheson Memorial Forests is a mixed oak stand located on the New Jersey Piedmont, and is believed to have remained uncut and unburned for >250 yr. Abundant canopy gaps and large fallen boles indicate old—age status. Patterns of detritus distribution appear different from younger or disturbed forests. Consequently, we studied organic matter and element content in 12 distinct detritus components, litter deposition, element ratios, element turnover rates, and latitudinal oak forest detritus affinities. Total detrital organic matter (ash—free) was 27.6 kg/m², with 86% located in the mineral soil horizons. Of the 3.8 kg/m² of detritus lying above the mineral soil, 71% was in fallen boles and large branches. Detrital organic matter exceeds aboveground biomass. In the forest floor and associated decaying wood, N was the abundant element (81 g/m²), followed by Ca (22 g/m²), Mg (11 g/m²), K (10.5 g/m²), and P (4.5 g/m²). The thin humus layer, averaging 0.5 centimetres thick, had a much higher total element content per square metre than older detrital components due to high element concentration values. Concentrations of each element increased as branch diameter decreased, and as leaf, branch, and bole litter decomposed. Organic matter:element ratios exhibited the same trend as carbon:element ratios, a decrease with decreasing branch diameter, increasing forest floor depth, and increasing bole decay. Average annual litter deposition (ash—free) was 616 g/m². Nitrogen, P, and K content of litter deposition was higher in summer than in autumn. Leaves returned the majority of each element followed by small branches and fruits. Forest—floor organic matter and element content increase along a latitudinal gradient of eastern oak forests. Organic matter and element turnover times of southern oak forests, including Hutcheson Forest, are similar, despite differing organic matter and element pools and deposition rates. One interesting aspect of detritus in an old, undisturbed forest is the pronounced role of detrital wood. Large decaying boles represent 10% of aboveground biomass and 9% of total detritus. These boles are a habitat and energy source for detritivores, influence seeding establishment and soil development, and affect hydrologic and biogeochemical cycles. Because of large carbon:element ratios and slow rates of decomposition, boles lying on the forest floor may exhibit a net accumulation and storage of elements until a critical carbon:element ratio is reached and net mineralization occurs. A time delay thus exists for element release which may provide regular elemental supply through time.